Preventing mold and mildew growth on composite decking

Mold and mildew can be a nuisance on any exterior building surface, regardless of the material. If the conditions are right, they will spawn on wood, plastic, concrete, metal and other surfaces. Mold formation is most prevalent in consistently wet, shaded areas. Spores from the natural environment are carried by the wind and commonly land on decks surfaces. It is important to note that the appearance of mold and mildew is a function of nature, not necessarily a deficiency with any of the material on which it grows.

Deck Cleaning
Periodic washing with soap/detergent and water will help remove surface dirt. This will also help prevent the buildup of pollen, debris and spores that can cause and accelerate mold/mildew growth (it also helps to remove deck mold). Caution: A pressure washer should not be used to “blast” mold/mildew or soils from a deck surface. The abrasive nature of the water stream can potentially damage the material by driving the spores into the material, which may create a moreMold on composite deck before/after cleaningchallenging problem to remediate. A pressure washer with a fan-tipped nozzle should be used only to lightly wet or rinse wood or composite deck surfaces. 

There are many deck wash and exterior cleaning products available at retail. It is important to make sure you use a cleaner specifically intended for your application. After selecting a product, be certain to read, understand and follow all instructions supplied by the manufacturer. Some cleaning products and inhibitors may be more effective than others, depending on the environmental conditions your deck is subjected to. Additionally, it is always a good idea to test the cleaner in a small, inconspicuous area prior to applying it to the entire deck.

Mold Inhibitors
As with deck washes, there are several mold-inhibitor products available from paint stores, hardware stores, online outlets and home centers to help prevent long-term mold/mildew growth. For any product selected, be certain to read, understand and follow all instructions provided by the manufacturer. Depending upon the environmental factors affecting your composite lumber or treated wood deck, some preventive cleaning products may be more effective than others. It may be necessary to try more than one product. For ongoing preventive maintenance, follow the manufacturer’s recommendation.

Comments for Preventing mold and mildew growth on composite decking

Name: D
Time: Wednesday, June 23, 2010

I'm re-building our patio picket fencing (cement base/floor) but am getting black spots overnight on a lot of the wood. I'm using my 15 year old cedar pickets (power sanded down to good new wood) + New cedar horizontal rails. All wood has been treated with Thompson's Water Seal and allowed to dry/sun cure for several weeks. The pickets have been yearly re-sealed for at least 13-15 years. Pickets look new after power sanding down to "new wood". One day after re-installing the fencing, most (old and new) cedar was covered with Black Dots. It seems to be growing each day (rain or shine) since then. Do you have any advise to help me out here? Is this Mold, Mildew, or what messing up the project? ANY help you have to offer will be greatly appreciated. Thank you dan The Thompson's Water Seal is from last year but been stored in the attic over winter. Can the Water Seal go bad with and be carrying some exotic fungus/mold/mildew?

Name: Chris Fox
Time: Wednesday, June 23, 2010

Dan, every indication would be that the black spots are a fungus growing on the cedar wood. Most cedar is resistant to fungus, but fungus can grow on virtually any surface given the right conditions. There also are many airborne fungi that attack wood under certain conditions. Many of the airborne fungi, once in the wood cannot be removed, but applying chemicals that stop entry can prevent them. Most mold/mildew can be removed by apply a mold inhibitor with a pump up sprayer. After it dries you can apply a water repellent and UV protective coating. If the conditions that fostered the growth don't change the fungus will probably return. Contact the chemical manufacturer to determine if there is a shelf life on the sealant. I suggest testing a mold-cleaning product, on a scrap piece of your cedar or an inconspicuous spot on the fence. If the mold/mildew remover is effective then the entire fence will need to be addressed, either by a thorough cleaning with mold/mildew remover or treatment with a mold inhibitor product

Name: john
Time: Tuesday, April 12, 2011

hi. i have 2 separate composite decks build by 2 different builders, 2 different periods. Both seem to get horribly black build up on the surface. i have repeatedly tried deck was and it down not get it up. I have power washed theses decks and they come up like new and last mose of the year. i live in NC. is there something i can seal these surfaces with after i power wash them to maintain the look?

Name: Chris Fox
Time: Tuesday, April 12, 2011

Hello John, thanks for your question. It sounds like your composite decks are in shady and possibly damp environments and around trees, which typically leave behind a film of dirt and mold spores. I would suggest cleaning your composite decks twice a year with a light detergent and a scrub brush, and be sure to thoroughly rinse after cleaning to ensure the majority of the dirt and spores are rinsed off the decking surface. Clean the composite decking and railing surfaces thoroughly in the spring and again in the late fall. Cleaning your deck twice annually will help reduce the buildup of mold spores. In the last stage of cleaning you composite deck you may want to consider applying a mold and mildew inhibitor / stain remover. There are products out on the market like Mold Off and Wet & Forget that could help minimize the occurrence of mold. As always, test any chemical on an inconspicuous area of your deck before applying to your entire composite deck surface. In addition, try pruning tress that overhang your deck area, as this will allow sunlight to shine through and help eliminate falling dirt and debris from the overhanging trees.

Name: Grayce
Time: Thursday, August 4, 2011

Spores are on composite decking, I use a paint brush to paint the deck with gallon of JoMax Deck Wash, work up a lather and after 10 /15 minutes, wash off with water hose, does a beautiful job when dried. Now the product is not available and Olympic Deck wash was recommended, next time I need it cleaned. I have a small Japanese Tree nearby, and will have to cut it down. I heard that mulch is responsible too. I would never build with composite again. I am going to experiment with one cup TSP powder, 3 quarts warm water and one cup bleach, recommended to use stiff brush.

Name: Chris Fox
Time: Friday, August 12, 2011

Grayce, some composite decking is better than others at resisting the effects of mold buildup. However, mold/mildew is know to grow on virtually any kind of surface from treated lumber to glass and metal. Decks with heavy tree cover/shade are more susceptible to buildup of spores which lead to dark mold/mildew spots and streaks.

I clean my Latitudes Composite Deck and Railing 2X per year with a mild detergent to help eliminate any buildup up of grime and spores from trees, etc.

Please be careful when using bleach as it may discolor your composite surface (to much bleach will fade the color).

Name: Dave
Time: Friday, February 22, 2013

Hey I have the composite deck from lowes and I tried the expensive corte clean. I looked at it and found that it wasnt good enough for the price i paid for it. I went hunting under the sink and tried the clorox clean up cleaner with bleach and applied it and lighly scrubbed it in and let it wait 10 min and wow that cleaned the spots off and cleaned the deck real nicely. Give it a try i went to family dollar and picked up a bottle under $5. So far the mold spots have not returned

Name: Chris Fox
Time: Thursday, February 28, 2013

Dave-Though this seems to have worked, it is concerning since it's basically bleaching your deck. According to the Clorox website, composite decking is not listed as an approved surface for this product, while they do have one that is.

I would not recommend bleach or any one particular product to clean a composite deck, but to visit a local lumber retailer where you purchased your composite decking to find an appropriate cleaning product for your deck. You may reference another of my previous blogs on this subject here:

Name: Kevin Senn
Time: Sunday, September 1, 2013

How do you rid composite decking of mold spots if the spores have penetrated the surface of the material?

Name: John Fitton
Time: Monday, September 9, 2013

Kevin - While it may appear that mold is penetrating the surface, odds are it is just surface mold. Surface mold can be taken care of by using a composite deck wash made specifically to reduce and prevent mold growth. Along with that it is important to make sure and wash your deck at least twice a season to prevent spore build up.

If, in fact the mold has penetrated the surface of the boards, check your warranty. Many composite products carry a warranty against deterioration from mold and other external factors. Then contact the manufacturer and alert them to your case. They should take the necessary steps to remedy the problem if warranty on the product was not void. Thanks for your question!

Leave a comment


Universal Forest Products is the nation's leading manufacturer and distributor of wood and wood-alternative products to the retail building materials, industrial packaging/components, manufactured housing, residential construction and commercial construction & concrete forming markets.